For certain types of manufactured objects it is advantageous to inspect the object for conformance to predetermined criteria without contacting the inspected objects. For example, nuclear pellets for fuel rods must be inspected following grinding for the presence of surface defects, for conformance to predetermined dimensions and for other anomalies which may adversely affect the performance of the pellets in the rod. To avoid contact with the cylindrical pellets an optical inspection may be made, preferably by means of a system whereby the entire process is automated. In order to inspect the cylindrical surface of the object of interest, a beam of light is projected onto the surface an photosensitive elements are utilized to detect light which is reflected back from the surface of the object. The photosensitive elements produce output signals representative of the amount of the reflected light incident thereon. Variations in these output signals are interpreted as corresponding to variations in the surface features of the object.
One requirement for the proper functioning of such apparatus is that the variations in the intensity of reflected light be produced only by the surface features of the object, rather than by variations of the intensity of light which illuminates the object. Such light sources as are commonly available for the intended purpose do not, as a rule, meet this requirement because the intensity distribution of the light provided often exhibits variations across the cross section of the light beam, as well as variations with time.
Another problem connected with this type of optical inspection technique arises from variations in the intensity of reflected light caused by variations in the reflective properties of the inspected object. Thus, a light beam reflected by a smooth surface which is shiny (that is, specular) travels substantially in a single direction away from the point, or locus, of reflection on the surface. There is relatively little scattering of light in random directions and thus a large proportion of the reflected light, commonly termed specularly reflected light, will reach the photosensitive elements. However, when a light beam is reflected by a smooth surface which is dull (that is, diffuse), more scattering of light in random directions will occur and less of the reflected light, termed diffusely reflected light, will reach the photosensitive elements.
When a light beam is reflected by a surface which is rough rather than smooth, i.e. a surface which contains irregularities such as cracks or pits, light will to a large extent be reflected in random directions, and it can be specularly or diffusely reflected. Here, the loci of reflection are no longer located on the surface, but at the walls and floor of the cracks or pits. These loci are randomly oriented and hence light is reflected in random directions. Thus, less light will be reflected to the photosensitive elements than even for a dull surface. In order for the inspection apparatus to operate effectively, it must be capable of distinguishing between the different types of conditions outlined above. Further, it must do so on a continuing basis since different surface areas will be inspected as the object advances past a viewing field.
A further requirement of optical inspection apparatus of the type under discussion, is that light be utilized in an efficient manner. Two types of light source are commonly employed, incandescent and coherent, and both provide light of relatively low intensity. Although high intensity incandescent light sources are available, they produce excessive heat which may be undesirable in the context of the inspection procedure. On the other hand, high intensity coherent light sources are prohibitively expensive. Thus, inasmuch as practical considerations dictate the use of low intensity light sources, light loss must be avoided by efficiently collecting reflected or transmitted light and directing it to the photosensitive elements. Further, the light beam must be properly focused to illuminate only the region which is viewed by the photosensitive elements. For a linear array of elements, such a region must therefore be linear.